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// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#ifndef STORAGE_LEVELDB_UTIL_CRC32C_H_
#define STORAGE_LEVELDB_UTIL_CRC32C_H_
#include <stddef.h>
#include <stdint.h>
namespace leveldb {
namespace crc32c {
// Return the crc32c of concat(A, data[0,n-1]) where init_crc is the
// crc32c of some string A. Extend() is often used to maintain the
// crc32c of a stream of data.
extern uint32_t Extend(uint32_t init_crc, const char* data, size_t n);
// Return the crc32c of data[0,n-1]
inline uint32_t Value(const char* data, size_t n) {
return Extend(0, data, n);
}
static const uint32_t kMaskDelta = 0xa282ead8ul;
// Return a masked representation of crc.
//
// Motivation: it is problematic to compute the CRC of a string that
// contains embedded CRCs. Therefore we recommend that CRCs stored
// somewhere (e.g., in files) should be masked before being stored.
inline uint32_t Mask(uint32_t crc) {
// Rotate right by 15 bits and add a constant.
return ((crc >> 15) | (crc << 17)) + kMaskDelta;
}
// Return the crc whose masked representation is masked_crc.
inline uint32_t Unmask(uint32_t masked_crc) {
uint32_t rot = masked_crc - kMaskDelta;
return ((rot >> 17) | (rot << 15));
}
} // namespace crc32c
} // namespace leveldb
#endif // STORAGE_LEVELDB_UTIL_CRC32C_H_
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